Background
I am working with a client on a large project which requires a custom networking chip to be designed to solve the data transfer requirements within the project. The network is intended to send small packets a few inches from one PCB to another over a single twisted pair cable. We will be designing and specifying the network protocol, and another company will be responsible for the silicon implementation.
I estimate that 20Mbps data rate between nodes will easily cope with the amount of data that needs to be sent, with plenty of head room should the amount of data increase in the future.
Problem
The client is asking me why I am specifying only 20Mbps. Why not something like 1Gbps? Wouldn't that be better? Intuitively, I feel that cranking up the data rate massively beyond what would be needed is a bad idea. Initially, I thought that the cabling would need to be shielded (which I don't want), but looking at the Ethernet cable categories, I see that Gigabit Ethernet can run on Cat 6 cable, which doesn't need to be shielded.
Other Constraints
- The project is desperately space constrained, and do we don't have room for things like magnetics, unless it's a very small component (0603 max).
- The cables need to be as slim and flexible as possible.
- The device will run from plug-in power, so there is no particular low-power requirement.
Question
What are the problems, in terms of silicon design, cabling, and anything else, that may be faced at 1Gbps, that wouldn't be nearly as bad at 20Mbps? Should I go with my client's suggestion of implementing the network at 1Gbps, or should I insist on implementing only what is required?
We're under strict NDA, so I can't give too many details about our requirements. But please leave a comment if clarification is needed.
Best Answer
A few reasons:
Power
Faster speed means more power. Not only do you need faster analog circuits, which will consumer more power, all your electronics surrounding them need to be faster. Your digital systems, your latches, clock management, etc. If you get that 1 Gbps by using multilevel signalling you now need better ADCs and DACs. You might need to start dealing with more complex filtering. You could start requiring FEC which also needs to keep up.
Chip size
Faster means more going on. You need better clock stability, which means bigger circuits. You need better timing, which means a more complex clock recovery system. You might need to switch to using DSP to do channel equalization. Your potentially needed FEC needs chipspace.
Environment sensitivity
If you switch from a few tens of megabaud to whatever is needed for gigabit, you will become far more sensitive to the environment. Small mismatches which might be unnoticeable at a few tens of MHz become resonant stubs at higher frequencies. Reflections might start causing intermittent performance. A nicked cable due to abuse over the years (I don't know the application environment for your product) might be fine for lower speeds, but cause poor performance when you go higher.
Design effort
I think it is obvious from all of the additional issues I discussed above that the time and effort of designing a faster communication link is significant. This alone should be enough of a reason.
EMI
Faster speed means meeting EMI requirements could be harder.